The nondipolar component of the geomagnetic field is likely to dominate during at least some geomagnetic reversals. The magnetosphere and auroral zones for a nondipolar field are more complex than for a dipole field. To begin to explore the nature of this complexity, we have studied the auroral zones appropriate to a pure quadrupole field. The quadrupole coefficients for the historical geomagnetic field were used in the study. The result is that such auroral zones are much more extensive than at present. They extend in association with two great circles from the tropics to midlatitudes in both north and south hemispheres. As a consequence, bombardment of the upper atmosphere by magnetosheath (polar cusp) and quasitrapped energetic magnetospheric (plasma sheet) particle is also much more extensive.

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It is shown that if the field directions (inclination and declination, or other combination of two independent angles) are completely known on the surface of the earth, the geomagnetic potential can be determined uniquely except an arbitrary multiplicative constant. On the other hand, when declinations only are specified on the surface there are infinitely many potentials which satisfy exactly the same boundary conditions. Such non-uniqueness seems also to be present for the cases of other incomplete data set composed of one angle only. The uniqueness theorem serves as the basis for spherical harmonic analyses of the paleomagnetic field.

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Magnetic domain observations were carried out on iron meteorites and on sputter-deposited thin films of meteoritic composition (10, 20% Ni-Fe), by two SEM techniques: 1) high voltage (30kV) backscattered electron mode and 2) low-voltage (2kV) secondary electron image. To confirm and complement these, 3) Bitter-powder patterns were also observed in reflected-light microscopy. High energy SEM magnetic contrast revealed parallel-oriented arrays of stripe domains in kamacite (k) regions, at 60° (for ‹111› face) or 90° (for ‹100› face) to their taenite (t) boundaries, depending on the crystal orientation. Magnetic domain widths ranged from 15-25μm in the Gibeon meteorite (finest octahedrite, 8% Ni-Fe, average k-bandwidth 0.2mm) to 40-80μm in the Odessa meteorite (coarsest octahedrite, 7.29% Ni-Fe, 3.3mm k-bands). Finer domain substructure (-10μm) was resolved by Bitter-pattern microscopy in Odessa, as magnetic grids in each k grain, whose orientation was discontinuous across boundaries. Based on low-energy SEM observations of Gibeon, it appears that each micro-kamacite grain in plessite regions is a single domain. By the same technique, 1-5μm stripe domains separated by ≤0.5μm Bloch walls were resolved in a 20% Ni-Fe film and 0.2-2μum domains with 90° “elbow”-walls, typical of Fe₃O₄, were observed in a laboratory-produced Fe₃O₄ film. The much finer scale of magnetic domain structure in meteorites, by comparison with their metallographic-compositional features, may explain the presence of stable paleoremanence in the coarsely crystalline iron meteorites.
The application of even higher-voltage (50kV) SEM to similar studies of terrestrial, lunar, and meteoritic materials promises to reveal the bulk domain-structure of their magnetic carriers and may thus afford a better understanding of their paleomagnetic record.

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Perturbation of the steady current by a spherical resistivity anomaly embedded in a uniform medium is investigated for three typical models representing an anomalous area caused by dilatancy in the crust. The current is spatially uniform for one model and approximately uniform in the vicinity of the anomaly for other ones. Changes in amplitude and direction of the electric field seem to be observable at measuring sites near the anomalous area if the area is located very close to the earth's surface. The results are applicable to the electric field induced in the crust by fluctuations of the geomagnetic field if the period of variations is not very rapid so that the induced electric field can be treated as a uniform one in the upper crust. Thus a possibility of detecting changes in resistivity associated with shallow earthquakes by measuring the magnetotelluric fields is suggested from a theoretical consideration.

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